The present invention relates to a sensor such as an electrostatic capacitance type pressure sensor or acceleration sensor and an electrode extraction structure and method.
An electrostatic capacitance type pressure sensor which detects the pressure of a medium to be measured from the displacement of a diaphragm includes a stationary electrode and movable electrode which are formed on the inner surfaces of a sensor main body so as to face each other through a cavity. This sensor is designed to measure the pressure of the medium by detecting a change in capacitance between the two electrodes accompanying the elastic deformation of the diaphragm (see Japanese Patent Laid-Open No. 6-265428).
As an electrode extraction structure for such an electrostatic capacitance type pressure sensor, a structure designed to extract a stationary electrode and movable electrode to the outer surface of the sensor main body which is located on the opposite side to the diaphragm side is known (U.S. Pat. No. 6,382,030).
FIGS. 6A and 6B show the electrostatic capacitance type pressure sensor disclosed in U.S. Pat. No. 6,382,030. An electrostatic capacitance type pressure sensor 1 includes a sensor main body 4 which is formed by superposing first and second substrates 2 and 3 and directly bonding them and a stationary electrode 5 and movable electrode 6 which are formed on the inner surfaces of the sensor main body 4 which face each other.
The first substrate 2 is made of a square plate-like member having a uniform thickness. The stationary electrode 5 is formed on the central portion of the inner surface of the first substrate 2. Three electrode extraction holes 7a to 7c which are through holes are formed in the first substrate 2 so as to be located outside the stationary electrode 5. Of these holes, the electrode extraction hole 7a is a hole for extracting the stationary electrode 5 to the outer surface side of the first substrate 2 via a connection pin 11A. The remaining two electrode extraction holes 7b and 7c are holes for extracting the movable electrode 6 to the outer surface side of the first substrate 2 via connection pins 11B and 11C.
Although the second substrate 3 is formed into a square plate-like member having the same size as that of the first substrate 2, a circular cavity forming recess portion 42 is formed in the central portion of the inner surface of the second substrate 3. An elastically deformable thin diaphragm portion 3A is formed on the central portion of the second substrate 3 in which the cavity forming recess portion 42 is formed. An outer peripheral portion surrounding the diaphragm portion 3A forms a thick stationary portion 3B. The stationary portion 3B is directly bonded to the outer peripheral edge portion of the inner surface of the first substrate 2. The cavity forming recess portion 42 is sealed with the first substrate 2 to form a cavity 21.
The movable electrode 6 on the second substrate 3 is formed on the bottom surface of the diaphragm portion 3A to face the stationary electrode 5. The movable electrode 6 is comprised of a circular sensing electrode 6A and a ring-like reference electrode 6B surrounding the sensing electrode 6A. The sensing electrode 6A and reference electrode 6B respectively have movable electrode extraction portions 9A and 9B extending from the diaphragm portion 3A. The distal end portions of the sensing electrode 6A and reference electrode 6B respectively form electrode extraction pad portions 10A and 10B which respectively face the electrode extraction holes 7b and 7c. 
A stationary electrode extraction pad portion 8 is formed on the inner surface of the second substrate 3. The pad portion 8 is not electrically conductive with the movable electrode 6 and formed outside the diaphragm portion 3A to face the electrode extraction hole 7a. Note that the portions of the second substrate 3 on which the pad portions 8, 10A, and 10B are formed are formed to have the same thickness as that of the diaphragm portion 3A.
The connection pins 11A to 11C are respectively inserted into the electrode extraction holes 7a to 7c of the first substrate 2. The connection pins 11A to 11C are connection pins for extracting the stationary electrode 5 and movable electrode 6 to the outer surface of the first substrate 2 (the surface on the opposite side to the surface on which the first electrode 5 is formed). The inner ends of the connection pins 11A to 11C are electrically and mechanically connected to the pad portions 8, 10A, and 10B, respectively, with solder paste 12.
The connection pins 11A to 11C are connected to the pad portions 8, 10A, and 10B with the solder paste 12 by the following method. The connection pins 11A to 11C whose insertion-side ends are coated with the solder paste 12 in advance are inserted into the electrode extraction holes 7a to 7c to bring the solder paste 12 into contact with the pad portions 8, 10A, and 10B. In this state, the solder paste 12 is heated and fused from outside the sensor main body 4, and is then cooled and hardened, thereby connecting the connection pins 11A to 11C to the pad portions 8, 10A, and 10B.
As a material for the stationary electrode 5 and movable electrode 6, a Pt/adhesion strengthening film, e.g., Pt/Nb, is used. As a material for the solder paste 12, a material with low wettability, e.g., Sn—Ag, is used. As a material for the stationary electrode extraction pad portion 8, an Au/barrier layer/adhesion strengthening film having high wettability with the solder paste 12, e.g., Au/Pt/Nb, is used. Since the movable electrode extraction pad portions 10A and 10B are made of the same material as the electrode material, an Au/Pt/Nb film 13 is formed simultaneously with the formation of the stationary electrode pad portion 8 to increase the wettability with the solder paste 12.
In addition, a semiconductor acceleration sensor is also known, in which interconnection members are formed by sputtering instead of soldering as a structure for extracting the stationary electrode 5 and movable electrode 6 to the outside (see Japanese Patent Laid-Open No. 6-160420).
In the semiconductor acceleration sensor disclosed in Japanese Patent Laid-Open No. 6-160420, interconnections are formed by metal deposition (sputtering) on the inner surfaces of through holes formed in a glass plate and predetermined portions near the holes, and are electrically connected to a stationary electrode and movable electrode, respectively.
In the electrostatic capacitance type pressure sensor disclosed in U.S. Pat. No. 6,382,030, in connecting the connection pins 11A to 11C to the pad portions 8, 10A, and 10B, respectively, by using the solder paste 12, if the wettability between the solder paste 12 and the pad portions 8, 10A, and 10B is low, sufficient bonding strength cannot be obtained. In contrast to this, if the wettability is high, the fused solder paste 12 may flow from the pad portions 8, 10A, and 10B to short-circuit the stationary electrode 5 and movable electrode 6 or cause connection failures between the pad portions 8, 10A, and 10B and the connection pins 11A to 11C. This reduces the degree of freedom in selecting materials.
In addition, as a necessary amount of solder paste 12 to connect the connection pins 11A to 11C to the pad portions 8, 10A, and 10B is heated and fused, it takes much time to cool and harden the solder paste, which has a low thermal capacity. This increases the thermal influence on the diaphragm portion 3A, resulting in residual stress in the diaphragm portion 3A. Obviously, when such residual stress is produced, the diaphragm portion 3A is not accurately deformed as the pressure of the diaphragm portion 3A changes, resulting in a deterioration in the measurement precision of the pressure sensor.
According to the semiconductor acceleration sensor disclosed in Japanese Patent Laid-Open No. 6-160420, since interconnections are formed by sputtering, the film-forming speed of the interconnections is very low, and the diffusion is large. It is therefore difficult to form a film having a large thickness (about 10 to 50 μm in general). In addition, the service life of a formed film is short. Since the movable electrode extraction pad portions 10A and 10B are spaced apart from the movable electrode extraction holes 7b and 7c through the cavity space, the interconnections formed on the inner circumferential surfaces of the movable electrode extraction holes 7b and 7c must be electrically connected to the movable electrode extraction pad portions 10A and 10B by sputtering a deposition metal in the cavity space between them. In this case, however, if the thickness of the cavity space is larger than the thickness of an interconnection, no interconnection can be formed in the cavity space. This method is therefore unsuitable for this case.